In general, in a process of treating raw water such as sewage and waste water discharged from industrial site (hereinafter, referred to as raw water), for conservation of water quality on discharging the raw water, the raw water is treated at a terminal end part of a discharge channel and thereafter discharged to the outside. If the raw water introduced from general home or industrial site undergoes chemical treatment while being stored in a circular sludge collector for a certain period of time, impurities mixed in the raw water sinks down to a bottom of the collector and sludge, i.e. precipitate that has sunk down to the bottom, is collected and moved by a scraper contacting with a bottom surface and then discharged to the outside through a sludge outlet, and overlying water other than the sludge is treated with chemicals and thereafter discharged to a river. The sludge settled on the bottom surface of the circular sludge collector is rapidly discharged to the outside and removed through the sludge outlet for preventing anaerobic degeneration of the sludge that drastically degrades the water quality over time, and thus purification of the raw water is smoothly achieved.
Such a circular sludge collector of the settlement basin includes a rotary unit for removing sludge having a circular body with an open top end, a raw water-supplying part configured to supply the raw water into the body, a center cage rotatably installed outside a raw water-supplying pipe of the raw water-supplying part, and a scraper joined to the center cage for scraping and moving the sludge while rotating; a cylindrical feed wall fixedly installed above the center cage and guiding the raw water so that the raw water flows downwards while contacting with an inner circumferential surface of a side wall; a turntable fixedly installed on an upper end portion of the center cage for rotation of the center cage; a drive fixedly installed at one inner end portion of a bridge for walking passage and having a driving motor provided with a reduction gear installed on a shaft for rotation of the turntable; and a sludge-discharging pipe connected to a sludge-receiving unit formed at a central part of the button surface described later of the body.
However, since there is a civil constructional problem that it is difficult to maintain a uniform gap between the scraper provided on one side or both sides of the circular sludge collector of the conventional settlement basin and a bottom part which is a concrete structure, a mechanical rotational force is driven using an elastic scraper made from flexible rubber or the like. Therefore, a problem arises of damage to the scraper due to non-uniform friction between the scraper and the bottom surface and of increase of cost for facility management due to excessively many machines or equipment structures. Moreover, another problem arises that part of the sludge is not removed from the sludge-discharging unit and the bottom surface, and there is yet another problem that the deposited sludge, which has not been removed, becomes anaerobic and thus is decomposed and cannot be smoothly discharged to the outside through the sludge-discharging pipe. Further, the maintenance such as repair or the like is difficult since the discharging pipe and inflow pipe have to be buried in a lower part of the settlement basin, which pipes are buried from a central part of the settlement basin structure to an underground outside the structure.